Computer Aided Design (CAD) applications are used to produce computer models of two and three dimensional objects as part of the production process for the actual physical device being modeled. The models frequently include multiple CAD part bodies which must be individually designed. A CAD part body is a computational model used by a CAD application to hold a solid or a sheet (open body with zero thickness) geometry. Once the designer is satisfied with the design, the actual physical device may be produced using the CAD model.
3D scanning captures physical geometry information for a three-dimensional object by gathering high resolution points representing the shape of the scanned three-dimensional object. The 3D scan data can be represented by either a set of points or dense triangular (or other shaped) meshes which cumulatively form a model of the scanned object. The model can be segmented into multiple groups referred to as regions. In a mesh model, the region is a mesh region that is a set of triangular (or other shaped) facets which can be arbitrarily defined by the user or can be automatically identified by a computer program. The computer program can also be designed to detect and group planar, cylindrical, spherical, conical, toroidal, or freeform mesh regions by estimating and tracing the curvature information. Once captured, the raw 3D scan data may be converted to a CAD part model for further processing to replicate or modify the design of the three-dimensional object. This procedure of capturing 3D scan data for a three-dimensional object in order to provide it to a CAD application so that the object may be replicated or redesigned is referred as reverse engineering.
During the reverse engineering process there are two major possibilities of losing the original scan data accuracy. The first possibility can happen during the editing processes for the raw 3D scan data 3D scan data is prone to be noisy so that the user needs to perform smoothing operations on the data to make it easier to directly calculate NC tool paths, build a RP (Rapid Prototyping) model or remodel a CAD part. However, a smoothing operation moves points in the scan data with the result that there occurs an accuracy loss compared to the original data. Another type of operation which leads to the loss of accuracy is decimation (or simplification). Decimation is a process to reduce the number of mesh facets by using bigger meshes when representing more planar (flat) regions. Decimation also modifies point coordinates from the original scan data.
The second part of the reverse engineering process where there is a substantial danger of losing the original scan data accuracy is during the designing of a CAD model replicating the raw 3D scan data geometry. A CAD part body is represented by a set of parametric surfaces whereas 3D scan data is represented by points or a set of triangular (quadric or other shaped) meshes. As a result there is always a disparity between the raw 3D scan model and the reverse engineered CAD model. The loss of accuracy can be interpreted as the deviation between the two models. Ideally, the user should be able to track and manage the accuracy loss throughout the whole reverse engineering processes from raw scan data editing to CAD remodeling. Unfortunately, the tracking and managing of the accuracy loss is not an easy task since the user is required to check the accuracy loss whenever the user edits the raw scan data and/or adds CAD features.